The present invention relates to a method of testing controllable switching devices arranged in a full bridge configuration.
If a crane holding a heavy load in a stationary condition is commanded to move, the brake then released and the motor drive engaged, an open or shorted silicon controlled rectifier (SCR) present in a bridge supplying rectified power to the crane motor could then cause insufficient torque to be provided to support the load. To avoid this situation torque proving in cranes is usually accomplished before releasing the brake by commanding a low torque from the drive. The current regulator loop is checked to see if a predetermined level of armature current is developed.
A prior art method of checking an SCR bridge monitored the voltage across each of the cells of the bridge by measuring the three AC input voltages as well as the two DC output rails to check for a collapsing voltage when the cells were fired into the motor filter. This approach requires extra voltage sensors and will not detect an open cell in parallel with a good cell in a dual bridge configuration.
It is an object of the present invention to provide SCR test of SCRs arranged in a full bridge configuration which does not require any additional voltage or current sensors than those required for controlling the drive.
It is a further object of the present invention to provide a drive with improved maintainability capable of identifying open or short circuited SCRs.
It is a still further object of the present invention to provide a method of SCR testing that will detect open or short circuited SCRs in a dual bridge rectifier.